This invention relates to an arrangement for frequency-modulating an oscillator by applying a modulation voltage to a varactor diode, and particularly to such an arrangement for providing a more linear relationship between the oscillator frequency and the voltage applied to the varactor diode.
Varactor diodes are commonly used for frequency modulating high-frequency oscillators. Typically, a varactor diode is connected into the frequency-determining circuit of the oscillator, and the oscillator frequency varies because the capacitance of the varactor diode is varied as a function of the voltage applied to it. Thus as the diode capacitance varies, the oscillator frequency also varies, but not necessarily in a linear manner with respect to the modulating voltage.
The basic problem is to compensate the inherent non-linearity of the varactor modulator, without affecting the efficiency, noise performance, frequency and tunability of the oscillator. Typical of previous solutions employed are those disclosed in U.S. Pat. No. 3,747,032, issued July 17, 1973 to inventors James A. Hall and Harry J. Peppiatt and is titled "Arrangement for Providing Improved Linearization of the Voltage-Frequency Characteristic of a Resonant Circuit Having a Voltage-Variable Capacity Diode". In this case the diode is coupled through a one-third wavelength transmission line to a resonant cavity. The capacitance of the diode is varied by the modulating voltage changing the resonant frequency of the cavity, and hence the resonant frequency of the oscillator. The transmission line causes the resonant frequency of the cavity to vary more linearly over a relatively wide band as a function of the modulating voltage. In coaxial frequency modulated oscillators which employ varactors, a first resonator in the form of a coxial cavity is connected to the collector transistor with a sliding short at the cavity which controls the rest frequency of the oscillator. A second resonator circuit is terminated by a sliding short on one end and a varactor on the other. The second resonator is capacitively coupled to the main resonator. The amount of coupling will determine the deviation and with the sliding short the linearity is adjusted.
The above described circuits are realized in mechanical configuration which require expensive manufacturing because of tight tolerances, fine threads and, for the coaxial circuit, gold plating of large areas to reduce the electrical loss of the cavities. Because air is used as a dielectric medium, the units tend to be large and bulky. They are also susceptible to vibrations and shock (microphonics). Further, the electrical arrangement required has the output coupling of the main resonator closer to the modulation resonator which makes the linearity very susceptible to changes by changing output coupling and load. Typically the linearity and frequency adjustments are highly interdependent.
In order to eliminate some of the above-mentioned drawbacks, oscillators and consequently their modulation circuits have recently been realized in microstrip form which generally reduces the size of such units. Typical of the state of the art are circuits described in "An RF Linear Modulation Circuit" by G. Bock and B. Walsh, IEEE-MTT-S International Microwave Symposium Digest, pp. 315-317, Ottawa, 1978; and "Computer Aided Design of Highly Linear, High Power Varactor Tuned Frequency Modulators" by E. Marazzi and V. Rizzoli, IEEE-MIT-S International Microwave Symposium Digest, pp. 88-90, Ottawa, June 1978.
In the first circuit, the modulation circuit is part of the output matching circuit which has the disadvantage of making the linearity very susceptible to the impedance of the load. The linearization circuit disclosed by Rizzoli is decoupled from the oscillator output but the frequency determining resonator, to which the modulation circuit is connected, is still realized in coaxial form. Neither of these circuits is meant to provide a significant linearity adjustment, unless the varactor bias is adjusted, thereby also changing the frequency. This makes it impossible to compensate for linearity slopes originating in other parts of a system, e.g., in the modulation amplifier of a transmitter.
The present invention solves the problem of physical size and interdependence of linearity adjustment and frequency. In the instant invention a very simple linearization circuit is completely realized in microstrip and the same varactor (modulation varactor) can be used for frequency control purposes (such as an automatic phase lock loop) since the varactor bias is not used for linearity adjustment and is essentially independent therefrom. Further, the linearity adjustment may be done remotely by adjusting the bias of a second varactor.